Many insulating boards are known for use as printed circuit boards and other applications. Conventional printed circuit boards are often found to have different dielectric constants when measured in different directions within the board. This probably results from the need to balance desired physical properties with the dielectric constant obtainable by conventional materials used for printed circuit boards. It has long been recognized that a truly isotropic printed circuit board, that is, having substantially the same dielectric constant measurement when measured in all or any directions, would be highly desirable. This has been accomplished in some cases by the use of alumina in printed circuit boards. However, the use of alumina often results in high cost boards which are difficult to machine and brittle thus detracting from their use in many applications.
It is an object of this invention to provide a composite polymeric insulating board useful as a printed circuit base board which insulating board exhibits isotropic properties with respect to its dielectric constant and loss tangent and which can be tailored to have any one of a wide variety of preselected dielectric properties.
Still another object of this invention is to provide an insulating board in accordance with the preceding object wherein the board can have incorporated therein a reinforcement material and an impregnating polymeric material resulting in a mechanically strong yet yieldable board having good mechanical properties for use as a printed circuit board.
It is still another object of this invention to provide methods of forming insulating boards in accordance with the preceding objects.
According to the invention an insulating board has a polymeric fibrous material forming a reinforcement layer with a predetermined dielectric constant. A second polymeric material is impregnated into the reinforcement layer and has a dielectric constant closely matched to the dielectric constant of the layer and preferably within a range of .+-.1 of the dielectric constant of said polymeric layer of fibrous material. The overall dielectric constant of the insulating board is isotropic and preferably lies in the range of from 2.2 to 20 when measured in any or all directions at a temperature of 25.degree. C. and at 60 cycles. Preferably the insulating board has a loss tangent in the range of from 0.0009 to 0.01 when measured at 8.5 kilomegacycles (GHz) at 23.degree. C. and 50% relative humidity.
The loss tangent or dielectric constant can be tailored to particular applications. Preferably the polymeric fibrous material is selected from the group of polymers consisting essentially of polypropylene, polyethylene and polytetrafluoroethylene and copolymers containing a major proportion of such olefin polymers. Preferably the polymer impregnated into and formed about the reinforcement is a polybutadiene or polybutadiene copolymer with a minor percentage of other monomers which are preferably vinyl unsaturated monomers.
Either the dielectric constant or the loss tangent can be made to be different from a pure polymeric impregnating material and polymeric fibrous reinforcement containing board by the addition of fillers to the impregnating material before polymerization. However, the fillers can only be used to preselect one or the other of the dielectric constant or the loss tangent.
When variations in dielectric constant and/or loss tangent are desired, fillers can be used to make such variation. Such fillers are inert and do not enter into the exothermic reaction during curing of the impregnated polymer about the fibrous material.
It is a feature of this invention that a wide range of dielectric constants and/or loss tangents can be obtained in an insulating board having good mechanical properties permitting a multitude of uses for the boards of this invention. The isotropic properties with regard to both the dielectric constant and loss tangent are extremely important when the insulating boards are used as bases for antennas or for conventional printed circuit base boards.